Metal halide vapor photochemical light sources



Dec. 16, 1969 P. D. JOHNSON 3,484,640

METAL HALIDE VAPOR PHOTOCHEMICAL LIGHT SOURCES Filed March 17, 1967 Z Mo m m W omfin w W Q ZHW vr n M J United States Patent f 3,484,640 METAL HALIDE VAPOR PHOTOCHEMICAL LIGHT SOURCES Peter D. Johnson, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Mar. 17, 1967, Ser. No. 623,945 Int. Cl. H01j 61/18 US. Cl. 313-210 8 Claims ABSTRACT OF THE DISCLOSURE or cadmium which emits molecular spectrum selectively peaked in the photochemically useful wavelengths. Means are provided for establishing a glow discharge within the envelope which vaporizes the halide, raises molecules thereof to an excited state from which they may emit their characteristic molecular spectra.

The present invention relates to vapor glow discharge lamps, and more particularly, to such lamps as are adapted to selectively emit radiation within the useful photochemical wavelength band of from approximately 2800 A.U. to 3400 A.U., that Wavelength band which is highly useful in the production of photochemical reactions in a variety of uses.

Prior art photochemical wavelength emitting lamps have primarily been mercury lamps, preferably low or intermediate pressure mercury lamps. Since, however, a substantial portion of an intermediate pressure mercury arc spectrum, for example, occurs at wavelengths substantially in excess of 3400 A.U. and even into the visible spectrum, the mercury arc is a rather inefficient photochemical light generator. This is because it is required that a substantial portion of the radiation thereof be wasted for photochemical purposes. Even though, in some instances, this radiation may not be harmful for the use intended, it nevertheless represents an energy loss. In those instances in which this radiation is harmful in the intended utilization, it is necessary to utilize filters to remove undesired wavelengths, thus adding to the cost and complexity of the photochemical light source as a finished product.

Accordingly, it is an object of the present invention to provide a simple and improved photochemically useful molecular gas light source.

Yet another object of the present invention is to provide a metallic halide vapor glOW discharge lamp having a highly concentrated band spectra within the photochemically useful wavelength band.

Briefly stated in accord with one embodiment of the invention I provide a lamp including an ultraviolet transmissive evacuable envelope having therein a pair of separated electrodes, a partial pressure of a starting gas and a quantity of a vaporizable halide, other than the fluoride, of a metal such as, for example, the halides of the metals zinc, cadmium, or mixtures thereof, in a sufficient quantity which upon partial or complete vapor- 3,484,640 Patented Dec. 16, 1969 Ice ization thereof under operating temperatures of the order of magnitude of 400 C., to establish within the lamp a partial pressure of metallic halide of approximately 0.1 to 10.0 torr and preferably of approximately 0.2 to 1.0 torr.

In operation, the application of a sufiiciently high voltage to the electrodes of the device causes the starting gas to become ionized and sustain a glow discharge which causes vaporization of the metallic halide contained therein in non-vapor form. After a suitable quantity of the metallic halide has been vaporized, a glow discharge of the excited metallic halide is established and the characteristic molecular band spectra of the excited metallic halide is emitted in such intensity as to produce a highly efficient photochemical light source within the wavelength band 2800 A.U. to 3400 A.U.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following detailed description taken in connection with the appended drawing in which:

FIGURE 1 illustrates, in vertical cross-section with parts broken away, a schematic embodiment of a photochemically useful wavelength producing lamp constructed in accord with the present invention, and

FIGURE 2 illustrates graphically the selective emission of a molecular spectrum of one metallic halide useful in the practice of the present invention.-

In FIGURE 1, a photochemically emissive light source constructed in accord with the present invention includes an exterior evacuable ultraviolet light transmissive envelope 1, which may for example be composed of pyrex glass or any similar high temperature, high strength ultraviolet transmissive glass, mounted upon and mechanically affixed to a screw type or equivalent contact-making base 2, having separate electric contact members 3 and 4. Envelope 1 includes therein an inner gaseous dischargecontaining envelope 5 which may, for example, be composed of any high temperature withstanding ultraviolet light transmissive material as for example quartz, Vycor, Lucalox (US. Patent No. 3,026,210), or high density yttria as disclosed and claimed in the copending application of R. C. Anderson, Ser. No. 582,755, filed Sept. 28, 1966, and assigned to the assignee of the present invention.

Inner envelope 5 is generally in the shape of a pinched sealed tubulation of generally circular cross-section, for example, and is hermetically sealed about appropriate lead and contact electrodes at pinched portions 6 and 7. Pinched portions 6 and 7 both serve to provide an hermetic seal to inner envelope 5 as it is fabricated from a substantially tubular member and to rigidly support the necessary lead-in and electrode members passing therethrough.

A pair of electric discharge electrodes 8 and 9, which may conveniently comprise coiled helical members of tungsten wire or thoriated tungsten wire or tungsten wire with a sliver of thorium contained therein or coiled-coil helices, as is well known to lamp technology, are located within interior envelope 5 at opposite ends thereof a sufficient distance apart so as to sustain a glow discharge therebetween with the application of appropriate voltages and with operating partial pressures of the metallic halide contained therein. In operation, the glow discharge is sustained by vaporized metallic halide and is responsible for the raising of the molecules of the halide to an excited state from which radiative transitions may occur resulting in the characteristic spectrum of the particular molecules involved and the emission of high intensity radiation within the photochemically useful wavelength band. Electrodes 8 and 9 are supported upon electrode lead members 10 and 11 respectively, which lead members are sealed through pinched regions 6 and 7 in hermetic seal. A starting electrode 12 may be located within one end of inner envelope 5 and is sealed through and hermetically sealed within region 6 of inner envelope 5. Starting electrode 12 is connected through a resistance 13 to a lead and support member 14 which is at the same potential with another lead and support member '15 both of which are connected to one contact member of connecting base 2. It will be appreciated however, that other means than starting electrode 12 may be utilized to start the lamp, as for example utilizing a pulse of high voltage to initially ionize the starting gas.

Inner envelope 5 is suspended within outer envelope 1 by means of a simple set of saddle clamp members 16 and 17 which are dependent from lead and support member 15 and which are securely mechanically fastened about the flattened portion of pinched regions 6 and 7 of inner envelope 5. Lower saddle clamp 16 is connected between support members 14 and 15; upper saddle clamp member 17 is connected between support member 15 and a suspended support member 18 which is connected with a collar 19 with a re-entrant nipple 20 within the upper portion of exterior bulb member 1, which serves to anchor the upper end of lead and support member 15. One discharge electrode 9 is connected to support member 15 and the other discharge electrode 8 is connected to a separate lead member 21 which is connected to the remaining contact member of screw base 2. The volume between envelopes 1 and 5 is evacuated or filled with a suitable ultraviolet transmissive inert gas or nitrogen at low pressure to minimize heat loss from envelope 5 to maintain a suitable operating temperature.

Envelope 5 contains a filler consisting essentially of a suitable partial pressure of starting gas sufficient to ionize and support a starting glow discharge when operating potentials are applied and which may conveniently be from 10 to 500 torr in pressure but which preferably is within the pressure range of 20 to 50 torr. It is preferred that xenon be utilized as the starting gas because of its low ionization potential, however other starting gasses may be utilized, as for example, argon or krypton. In addition to the xenon starting gas, the envelope contains a quantity 22 of a vaporizable halide (other than the fluoride) of a metallic salt which may conveniently be zinc or cadmium, although other volatilizable halides may be used, which halide is suitable from the point of view of vapor pressure characteristic, spectral emission of the molecular gas when in the vapor state, excitation potential and suitable operating temperature. In accord with the present invention I prefer to utilize zinc chloride as the metallic halide because of the ideal vapor pressure, wavelength, spectral emission, and other characteristics thereof. The quantity of halide present is sufficient to provide under operating temperature and voltage conditions a suitable partial pressure thereof to sustain a photochemically useful light emitting glow discharge within envelope 5.

Assuming the use of zinc chloride, a suflicient quantity thereof is added to produce under operating temperature conditions (minimum inner bulb wall temperature approximately 300500 C. and preferably approximately 3804S0 C.) to produce a quantity of vaporized zinc chloride which upon excitation by the discharge is effective to emit the desired intensity of the excited zinc chloride radiation. Under these conditions, the partial pressure of the vaporized zinc chloride present within the envelope rises to a value of approximately 0.1 to 10.0

torr, but is preferably maintained within the range of 0.2 to 1.0 torr. It will be appreciated, however, that only a portion of the zinc chloride is volatilized, since an excess prevents depletion of the halide by usual clean-up processes. This may readily be accomplished in a typical envelope having a volume of 25 cm. by adding approximately 50 to milligrams of zinc chloride salt. Operating temperatures of zinc bromide and zinc iodide do not markedly differ from those utilized in the embodiment of the invention utilizing zinc chloride as the emitting specie.

In operation, a voltage, which may vary from approximately 40 to 200 volts and which varies with the partial pressure of the metallic halide desired under operating conditions, is applied between discharge electrodes 8 and 9. A substantially similar voltage is applied between starting electrode 12 and discharge electrode 8 causing a glow discharge to be initiated in the starting gas sufficiently heating the zinc chloride or other metallic halide to cause vaporization thereof sufficient to cause a high concentration of metallic halide molecules within the glow discharge to facilitate excitation thereof and radiation therefrom. The glow discharge may have a current of approximately 25 to 200 milliamperes for the aforementioned voltages, the current being maximum when the voltage is minimum and vice-versa. When the aforementioned conditions have been satisfied and the glow discharge is established between the discharge electrodes and volatilization and excitation of the halide occurs, the spectrum of the zinc halide molecular emission is evident.

FIGURE 2 of the drawing illustrates the characteristic molecular spectrum of zinc chloride under conditions present in the operation of lamps constructed in accord with the present invention. Photochemically useful wavelength emission of the molecular gas resulting from volatilization of zinc chloride is vastly different from the line spectra that is characteristic of atomic specie as radiators. Whereas the spectra of atomic specie contain narrow lines representative of radiative transitions between various atomic energy states, the emission of an excited molecular gas is caused by the radiation emitted by one or more electronic transitions modified by closely related vibrational and rotational energy level structure. The net results of this complex emission, wherein the composite emission of a particular electronic transrtlon is modified by the presence of numerous vibrational and rotational states, results in a band structure and a very wide, and often highly peaked, emission spectrum such as that illustrated in FIGURE 2. Since the energy that is available for use by virtue of such radiation is measured by the integrated area beneath the curve such as that illustrated in FIGURE 2, it is readily apparent that the emission of a molecular gas, provided that it be in the desired wavelength region of the electromagnetic spectrum, inherently contains a far greater amount of energy and is potentially a much more selective and efficient light source than the emission of an atomic specie which relies upon a plurality of discrete lines, even though these lines may be very intense and even subject to collision line broadening.

From FIGURE 2, it may readily be seen that the zinc chloride radiation peaks at a value of approximately 3065 A.U. under the aforementioned conditions of glow current, wall temperature and halide partial pressure. A similar plot for the molecular radiation of zinc bromide under similar conditions of important parameters is the result of peaks at approximately 3070 A.U. and 3110 A.U. The spectrum of zinc iodide within the useful photochemical range is the result of peaks at approximately 3280 and 3320 A.U. Cadmium bromide, when the minimum inner envelope wall is maintained at a temperature of approximately 600 to 700 C., shows a band spectrum having a peak at approximately 3175 A.U. Cadmium chlolide within the envelope operated at approximately 600 to 700 C. exhibits strongly peaked band spectra at 3080 A.U. and 3180 A.U. Cadmium iodide, when operated in devices in the present invention, but with the inner bulb wall temperature maintained at a minimum of 400 to 550 C., exhibits a band spectra with a strong peak at 2385 and another at 3380 A.U. All of the foregoing spectra result from transitions of the monohalide as for example ZnCl, which exists constantly in an excited state. This does not imply, however, that the radiation is always the monohalide.

In one specific embodiment of the invention a 400 W. mercury arc discharge lamp envelope having an approximate volume of one litre made of pyrex glass enclosed an inner envelope of fused quartz having the shape illustrated in FIGURE 1, having an OD of 20 mm. and an ID of 18 mm. The electrodes were of a coiled coil of thoriated tungsten separated by a distance of cm., 75 milligrams ZnCl and 20 torr of xenon comprised the lamp filling. A voltage of 75 volts was applied to the electrodes and a glow discharge current of 50 ma. resulted in a high intensity of ultraviolet light peaked at 3065 A.U. substantially as illustrated in FIGURE 2 of the drawing. The intensity of this wavelength of ultraviolet light was approximately twice as bright as the intensity in this region obtained from a conventional 400 watt intermediate pressure mercury lamp of the same configuration.

While the invention has been set forth herein with respect to certain specific examples and particular embodirnents thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vapor glow discharge photochemically useful lamp comprising (a) an ultraviolet light transmissive evacuable envelope (b) a pair of discharge electrodes disposed within said envelope and defining therebetween a discharge path (c) a partial pressure within said envelope of an inert gas sufficient to support a starting glow discharge upon application of operating potentials to the electrodes of said lamp ((1) a filling within said envelope consisting essentially of a quantity of a vaporizable metallic halide, other than a fluoride, of a metal selected from the group consisting of zinc and cadmium sufiicient upon vaporization thereof to establish within said envelope a partial pressure of said vaporizable halide operative to support an electrical glow discharge between Said discharge electrodes (e) said electrical glow discharge being effective to raisesaid metallic halide to excited molecular states to provide high intensity efi'icient photochemically useful molecular spectral emission therefrom.

2. The lamp of claim 1 wherein the partial pressure of starting gas is approximately 10 to 500 torr and the partial pressure of metallic halide under operating conditions is approximately 0.1 to 10.0 torr.

3. The lamp of claim 2 wherein the starting gas partial pressure is approximately 20 to torr and the metallic halide partial pressure is approximately 0.2 to 1.0 torr.

4. The lamp of claim 2 wherein the starting gas is xenon.

5. The lamp of claim 1 wherein the metallic halide is zinc chloride and during operating conditions the coldest portion of the lamp envelope wall is approximately 300 C. to 500 C.

6. The lamp of claim 5 wherein the coldest portion of the lamp envelope wall is approximately 380 C. to 450 C.

7. The lamp of claim 5 wherein the lamp operates under glow discharge conditions at a voltage of approximately 40 to 200 volts.

8. The lamp of claim 5 wherein the lamp operates under glow discharge conditions with a discharge current of approximately 50 to 200 ma.

References Cited UNITED STATES PATENTS 2,765,416 10/1956 Beese et al. 313l3 3,234,421 2/1966 Reiling 3l325 3,237,041 2/1966 Barnes 313-228 X JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, Assistant Examiner US. Cl. X.R. 

